1. Field of the Invention
The present invention relates to ring halogenated vinyl aromatic monomers used as flame retardants in vinyl aromatic based polymers, especially such styrene monomers and vinyl toluene monomers. More particularly, the present invention is directed to a process for preparing ring halogenated vinyl aromatic monomers from ring halogenated bromoethylbenzene or bromoethyltoluene using a strong aqueous alkali base, poly(ethylene glycol) catalyst and certain types of amine stabilizer.
2. Description of the Prior Art
The problem of the flammability of polymer compositions is well-recognized. A variety of compounds exist that provide satisfactory flame resistance, smoke suppression and self-extinguishing properties.
Vinyl aromatic polymers, especially styrene based polymers make up a significant part of the plastics industry. Polymers having styrene as part of their structural makeup are for example polystyrene, styrene-butadiene copolymers, ABS, SAN, thermosetting polyester resins and copolymers of styrene with acrylate and maleic monomers, and many others. Polymers having methyl styrene or vinyl toluene as part of their structural makeup are for example para-methylstyrene. A deficiency of these styrene or vinyl toluene based polymers however, is their flammability. Consequently, there is an ever increasing awareness and interest in providing polymers which possess flame retardance.
The article entitled "Elimination Reactions of Polyhalopropanes under Emulsion Catalytic Conditions to give Halopropenes" by Wang in Synthesis June, 1982, pages 494-496 mentions the use of sodium hydroxide for the dehydrohalogenation of haloalkanes. Certain phase transfer catalysts are also identified. In "Modified Crown Ether Catalysts. 3. Structural Parameters Affecting Phase Transfer Catalysis by Crown Ethers and a Comparison of the Effectiveness of Crown Ethers to That of Other Phase Transfer Catalysts" by Stott et al in J. Am. Chem. Soc., Vol. 102, No. 14, 1980, pages 4810-4815, the effectiveness of poly(ethylene glycols) as phase-transfer catalysts is taught. In another article entitled "Poly(ethylene glycols) are Extraordinary Catalysts in Liquid-Liquid Two-Phase Dehydrohalogenation" by Kimura et al, J. Org. Chem. 47, 2493 (1982), the benefit of using poly(ethylene glycols) as catalyst in dehydrogenation reactions was recognized.
It was noted in U.S. Pat. No. 4,292,453 that when a reaction product containing about 5-10% dibromostyrene is stored for several hours before final workup some of the monomer polymerizes. The patent recommends the use of a nitrite salt as a monomer stabilizer during the reaction to reduce the amount of polymerization of bromostyrenes.
U.S. Pat. Nos. 4,338,474 and 4,343,956 recognize the use of a number of amines as stabilizers for dibromostyrene. These patents teach the addition of an amine directly to the dibromostyrene. The amines are not present during the synthesis of the dibromostyrene but merely function to prevent polymerization during storage.
It has now been discovered that by adding an aromatic amine polymerization inhibitor directly to the reaction during the preparation of ring-halogenated vinyl aromatic monomers along with a poly(ethylene glycol) catalyst and a strong aqueous alkali base that polymerization is inhibited and an increased yield of the ring-halogenated vinyl aromatic monomer is obtained.